Chris Walsh

Ecology of estuarine caridean shrimps

Summary

This study examines the factors affecting the distribution and abundance of epifaunal caridean shrimps in seagrass meadows of the Hopkins River estuary in south-western Victoria, Australia, and investigates the life history patterns which permit the freshwater Paratya australiensis to utilise estuarine habitats.

The epibenthic carideans were surveyed in four seagrass meadows spanning the length of the estuary.A qualitative survey of two meadows and a quantitative survey of four were each conducted for one year.Three species were collected:(a) the marine Palaemon serenus occurred only near the mouth over summer and autumn, when salinities were high and flows low; (b) the marine/estuarine Macrobrachium intermedium was the most common species occurring in all parts of the estuary, most commonly in the two more downstream meadows; (c) P. australiensis occurred throughout the estuary in spring, after the annual flood, and occurred in the upper part of the estuary in all seasons.

In contrast to other studies of M. intermedium in more marine locations, adults were least abundant in December and January, when salinities were high, and most abundant in April and May, with fluctuations in numbers being most pronounced at the upstream meadows.Juveniles were present from December to July.Adult P. australiensiswere most abundant from September to November, when salinities were low, disappearing from the estuary in December or January, as salinity rose, and juveniles were present from November to August, with a sharp peak in abundance (over 1000 m^-2) in December in the most upstream meadows.This is the first record of significant penetration of P. australiensis into an estuarine environment.

Canonical correlation analysis identified physical factors (salinity and temperature), driven by hydrological patterns, as the overriding environmental variables associated with shrimp abundance, while vegetative structure, and depth within meadows were less important.M. intermedium only showed a strong association with seagrass biomass at times of lower available seagrass biomass.M. intermedium adults tended to be associated with deeper water and P. australiensis adults with shallow water, apparently partitioning the seagrass meadows according to depth in November and December, just prior to the disappearance of adult P. australiensis from the estuary.

Salinity tolerances for each species reflected their distributions and reinforced the importance of salinity and temperature patterns in determining distributions.The upper 96 h LC₅₀ for P. australiensis was 27.6 at 10C declining to 19.2 at 28.5C, whereas M. intermedium and P. serenus were capable of surviving indefinitely in seawater.P. australiensis could survive indefinitely in freshwater, while the lower 96 h LC₅₀ at 20C for M. intermedium lay between 0 and 1, and was 7.0 for P. serenus.

Laboratory experiments showed competitive interactions between adult M. intermedium and P. australiensis for deeper positions in aquaria, consistent with patterns observed in the estuary.No interaction in relation to vegetative cover was detected experimentally.Competitive interactions were probably of secondary importance in determining shrimp distributions: a proximate mechanism that separated species in response to changes in physical factors.

Growth rates were faster and maximum size reached was greater in females than males of both M. intermedium and P. australiensis.Growth rates of M. intermedium were comparable to those reported previously, although faster than those recorded in Tasmania.Neither species showed variation in growth rates between sites within the estuary.

Many large ovigerous P. australiensis migrated into the estuary after peak discharge, skewing the sex ratio in the estuary.The population of P. australiensis just upstream of the estuary was dominated by males, suggesting they were less likely to migrate into the estuary than females.Ovigerous P. australiensis occurred in the estuary from September to December, after which no adults were found.Upstream of the estuary, breeding was more extended, lasting from July to April.Ovigerous M. intermedium were found in the lower section of the estuary from October to March, but did not occur in the upstream part of the estuary until November or December, when salinity had increased.It is probable that this contraction of breeding period in the upper section of the estuary results from physiological response to salinity rather than as a result of interspecific interaction.No ovigerous P. serenus were found in the estuary.

While no P. serenus larvae were collected in the estuary, high densities of both P. australiensis and M. intermedium larvae were collected.P. australiensis was an early coloniser, compared to other large plankters.Although a few P. australiensis larvae were present in the estuary as late as May, almost all larvae that occurred in the estuary did so during November and December, 6-10 weeks after peak discharge. Larvae were retained throughout the estuary, at the head of the newly intruding salt wedge as discharge declined, concentrating in the deep saline layer. M. intermedium larvae occurred in comparable densities to P. australiensis at their peak, but one to two months later, at all sites except the most upstream.They were present in the estuary until April or May.Their occurrence was less closely linked to the deep saline layer.The deeper part of the salt wedge was generally anoxic when M. intermedium larvae were collected, and they tended to be associated with sites with low surface flows and surface salinities >10.

Recruitment of P. australiensis juveniles to the seagrass meadows of the estuary was concentrated around November and December, after which numbers declined dramatically.Maximal juvenile densities were recorded in the two upstream sites, despite peak larval densities being comparable throughout the estuary, suggesting post-settlement migration upstream.A trapping program corroborated the hypothesis of juvenile migration out of the estuary in December.Two cohorts of M. intermedium juveniles recruited to the estuary from larvae in summer, but at least some juveniles migrated from adjacent coastal waters.

Post-larval migration was the sole mechanism determining the abundance of P. serenus in the Hopkins River estuary, and was at least as important a factor in determining the abundances of P. australiensis and M. intermedium as direct recruitment from planktonic larvae in the estuary.This challenges recent speculation on the importance of the distribution and availability of competent larvae in determining the distribution and abundance of epifauna in seagrass meadows.

The larval development of P. australiensis consists of eight stages and lasts between 28 and 45 days in the laboratory.Stages I to IV were regular, but stages V to VIII were irregular with 'mark-time moults' and a 'skipped stage' common.Metamorphosis occurred after 7-12 ecdyses.This development is intermediate between the abbreviated development of Caridina spp. and the extended development of other atyids.The larval development of P. serenus was typical of other Palaemon spp., consisting of 8-15 ecdyses and lasting between 27 and 62 days in the laboratory.The larval development of M. intermedium lasted between 17 and 49 days in the laboratory and involved 6-11 ecdyses.

Tolerances and optimal salinities of larvae of the three species reflected their distributions.Larvae of all three species were euryhaline, apparently capable of development under estuarine conditions.Both M. intermedium and P. serenus developed to metamorphosis in salinities from 15 to 35, and P. australiensis was capable of developing to metamorphosis in salinities from <1 to at least 15.The 96 h LC₅₀ of P. australiensis larvae was 36.7 at around 20.5C, considerably higher than for juveniles and adults.M. intermedium was the most euryhaline species, with faster larval development and less ecdyses at a salinity of 25 than at 35.Larval development of P. serenus was optimal in sea water.

The horizontal location in the estuary of P. australiensis larvae is probably maintained by persistence in the salt wedge which has no (or nett upstream) flow.Some movement of larvae upstream with the intruding salt wedge was evident from stage-frequency analyses.Vertical migrations into the fresh layer when it was flowing downstream were rare, but the migrations occurred more commonly on flood tides with no surface flow.Diurnal vertical migrations were evident within the salt wedge, and in a deep pool above tidal influence.

A survey of south-eastern Australian estuaries in December 1990 showed P. australiensis larvae present in high densities in all stable, open, well-developed, salt-wedge estuaries where adults were abundant.Adults were most abundant in low salinities among submerged leafy macrophytes.At least some larvae, juveniles or adults were present in all Victorian estuaries sampled.

Estuarine and fresh reaches of three neighbouring rivers, the Hopkins, Curdies and Gellibrand, were sampled monthly for adult P. australiensis over a single breeding season.In all rivers, large females occurred more commonly in estuaries than upstream in the earlier part of the breeding season, resulting in a greater proportion of ovigerous females in estuaries until December or January, when smaller females at upstream sites became sexually mature, and the proportion increased.

Egg number was correlated with female size while egg size was not.Early in the breeding season, within each river, egg size was larger, and (size-specific) egg number smaller in estuarine locations than in upstream locations.A trade-off between egg size and egg number resulted in no difference in total (size-specific) reproductive investment between locations.Reproductive investment tended to decrease at some locations over the breeding season , and this decrease was a result of decreased egg size in most cases.The decrease in reproductive investment probably reflected reduced food availability for the adult, while the reduced egg size was probably a response to improved conditions for larval development.In the Hopkins River, larger egg size at upstream sites was reflected in larger early stage larvae.These larvae developed an adult telson form at an earlier stage than estuarine larvae, which may be an adaptation to flow in riverine conditions.Later stage larvae were larger in the estuary, suggesting more favourable conditions for larval development.

Allozyme electrophoresis showed the P. australiensis populations in each of the three rivers to be distinct, with no significant difference between November and February at any site.There was no significant difference in allozyme frequencies among any of the Hopkins River sites, but upstream and estuarine locations in the Curdies and Gellibrand were significantly different.Although some of the differences in reproductive traits within catchments may have been due to genotypic differences, plastic responses to environmental cues within each catchment appear likely to explain the observed trade-offs between egg size and number, and decreases in egg size over summer.

It is proposed that P. australiensis successfully inhabits and reproduces in both estuarine and freshwater environments by plastic response to environmental conditions.Recruitment to estuaries is dependent on the presence of suitable adult, littoral habitat, and a stable salt wedge to allow the retention of planktonic larvae.Estuaries are important sites of recruitment for P. australiensis, potentially allowing an extra brood each year in addition to later riverine recruitment.The first estuarine brood of the season could constitute a large part of the total fecundity of each P. australiensis female.The discovery of euryhaline larvae and estuarine recruitment in P. australiensis permits new models of dispersal and differentiation for Paratya species of the western Pacific rim.